We just released a Web API Test Sandbox at the ReportingCloud portal. This sandbox can be used to test Web API calls with your account data and your template storage (so not a completely isolated sandbox per definition) that doesn't count against your document quota.
The following screenshot shows the sandbox in action for the merge endpoint:
The overview page also lists some sample Web API calls and in the Web API Endpoint Reference, each endpoint description shows a button to launch the test sandbox for that endpoint:
Happy coding!
